FIELD OF THE INVENTION
The invention belongs to the automotive field. Specifically, the invention relates to an antitheft device for a motor vehicle with which doors are locked or unlocked or an electronic immobilizer is selectively released.
A prior art antitheft device--described in German patent No. DE 43 29 697 C2--has a vehicle-mounted transceiver unit which can transmit and receive signals. If a user wishes to enter the vehicle, a challenge/response dialog is triggered by activating a trigger switch. A challenge signal (also referred to as a request signal, question signal, or interrogation signal) is thereby transmitted from the transceiver unit to a code transmitter which is carried by the user. The latter automatically transmits back an encrypted response signal if it has previously received the challenge signal. In the motor vehicle, the response signal is compared with an expected reference signal and, if the two correspond (=successful authentication), the doors are locked or unlocked. In such an antitheft device, however, the response signal may be recorded illegitimately by a third party with a recording device. If the third party then activates the trigger switch and subsequently transmits the previously recorded response signal, the doors are unlocked even though the unauthorized person is in the possession of the code transmitter. The unauthorized person can thus relatively easily enter the vehicle and also use it.